1. Field of the Invention
The present invention generally relates to inductance device more particularly, to an inductance device in which an inductive reactance component is surely attenuated at an intended desirable frequency.
2. Description of the Related Art
To prevent electromagnetic interference (EMI) in electronic equipment, an inductance device is usually incorporated into an electric signal line. The impedance of the inductance device includes a resistance component for attenuating noise and a reactance component for reflecting noise.
To reduce EMI, it is desirable to use an inductance device having a large resistance component and a small reactance component in a frequency band for noise removal.
Such an inductance device is disclosed in Japanese Utility Model Laid-open Pub. No. 4-130409.
The configuration of the inductance device disclosed in Japanese Utility Model Laid-open Pub. No. 4-130409 is shown in FIGS. 1 and 2. FIG. 1 is an outward appearance view. FIG. 2 is an exploded view.
In FIG. 1, the body (1) of the inductance device includes a magnetic body (1a) having a rectangular parallelepiped shape and conductive members provided inside the magnetic body (1a). The conductive members are connected to external electrodes (2, 3) formed at both longitudinal edges of the magnetic substance body (1a).
The conductive members provided inside the magnetic substance body (1a) are formed as shown in FIG. 2.
That is, the inductance device body (1) is formed by stacking magnetic material sheets (12a-12f) including internal conductors (11a-11f) and a magnetic material sheet (12g) including a closed loop internal conductor (11g). The internal conductors (11a-11f) are connected to each other in the form of a spiral by a through hole (14) to form a coil.
Moreover, the external electrodes (2, 3) are connected to both edges of the coil composed of internal conductors (11a-11f).
The above-described inductance device is thus composed of the coil formed by internal conductors (11a-11f) in the form of a spiral, the internal conductor (11g) that is located near the coil and is insulated from the coil and forms a closed loop to surround the central axis of the coil, the magnetic body (1a) which is formed by the magnetic material filling a fixed space including the internal conductor (11a-11g), and a pair of the external electrodes (2, 3) connected to the both edges of the coil and formed outside the magnetic body (1a).
According to the inductance device disclosed in said Utility Model Laid-open Pub. No. 4-130409, magnetic flux occurring in the coil induces an electric current to flow through the closed loop internal conductor (11g) which is insulated from the remainder of the coil magnetic flux occurring in the coil.
As a result, the resistance component of the inductance device increases, because this electric current is converted to heat by the resistance component of the conductive member which forms internal conductor (11g).
Moreover, the reactance component of the inductance device decreases because a reverse magnetic field occurs in response to the electric current which flows through the closed loop internal conductor (11g).
By these operations, noise can be removed and the occurrence of EMI can be prevented, because it is possible to set the value of resistance component R larger than the value of the reactance component (inductive reactance X.sub.L, capacitive reactance X.sub.c of the inductance device in a frequency band where it is easy for the noise to occur, e.g., the frequency band from 10.08 MHz to 1000 MHZ as shown in FIG. 3.
To make the resistance component R larger than the reactance component (X.sub.l, X.sub.c) of the inductance device, the internal conductor (11g) must be formed for the effect of the above-mentioned closed loop (that is the internal conductor (11g)) to occur at a frequency that maximizes the reactance. The frequency must be in a frequency band where the absolute values of both components are equal.
However, a method of obtaining an EMI reduction effect at a specific frequency has not been known in the prior art.
Therefore, the prior art needs considerable time to design the inductance device having the above-mentioned EMI prevention effect, because it is necessary to repeat experimental productions by cut and try methods to get a characteristic for the purpose.
Moreover, when the closed loop (the internal conductor (11g)) is placed in the center of the coil, where the coil magnetic field is the strongest, in order to obtain more improved closed loop effect, there is a problem that the shape and size of the closed loop are limited by the coil and then the degree of freedom in design decreases in the prior art since the closed loop is insulated from the coil.